With the need for reducing costs or times associated with business trips, more companies are moving towards transmission systems to have teleconference or videoconference among remotely located offices via a communication network. The transmission systems allow transmission of image data or voice data among a plurality of transmission terminals that are remotely located from one another through a communication network such as the Internet to facilitate communication among the plurality of transmission terminals. Before initiating communication among the plurality of transmission terminals, the transmission systems usually authenticate the transmission terminal, for example, by using an authentication system that authenticates the transmission terminal based on identification information received from the transmission terminal.
For example, the recent transmission systems encrypt the identification information of the transmission terminal with a terminal private key, and send the encrypted identification information to the authentication system together with information for identifying a terminal public key that is paired with the terminal private key. The authentication system extracts a terminal public key using the information for identifying the terminal public key, and decrypts the encrypted identification information using the extracted terminal public key. While this suppresses the possibility of identity theft, the authentication system needs to manage the association between the identification information of the transmission terminal and the information for identifying the terminal public key. Especially when there are a large number of transmission terminals, or information of the transmission terminal is frequently updated, it has been cumbersome for the authentication system to keep updated the identification information and the public key information of each of the transmission terminals.